An existing flat type strain wave gearing apparatus is equipped with a rigid first internal gear, a rigid second internal gear arranged side by side with the first internal gear, a flexible planetary gear arranged on the inner peripheral side of the first and second internal gears and deflected in the radial direction so that a swollen portion thereof is brought into mesh with the first and second internal gears, and a wave generator fitted into the flexible planetary gear and configured to deflect the flexible planetary gear. In this flat type strain wave gearing apparatus, when the wave generator rotates, an ellipsis drawn by the flexible planetary gear continuously varies due to a cam action; utilizing this, the partial mesh-engagement position of the internal gear and the flexible planetary gear is moved in the circumferential direction, whereby there is generated a relative rotation between both internal gears in accordance with the difference in number of teeth between the first internal gear and the second internal gear.
In this flat type structure, the overall configuration is compact, and the flexible planetary gear is of a simple cylindrical configuration, so that no excessive stress is applied during rotation, making it advantageously possible to maintain a sufficient configuration retainability while maintaining flexibility.